The long term-goal of our laboratory is to investigate novel approaches to prevent HIV transmission by the use of microbicides. For this purpose we have developed and implemented a novel small animal model where human stem cells are used to reconstitute the hematopoietic system of immunodeficient mice. In these humanized mice (designated BLT to represent the fact they are generated from a bone marrow transplant of mice previously implanted with a piece of autologus human fetal liver and thymic tissue) there is systemic reconstitution with human hematopoietic cells in all hematopoietic and non-hematopoietic tissues tested. As shown in the Preliminary Results section, these humanized mice are susceptible to intrarectal infection with HIV-1. Infection results in plasma antigenemia and progressive depletion of human CD4+ cells from the peripheral blood. We show that human CD4 T cell depletion is systemic and most dramatic in the human thymic organbid where double positive thymocytes are depleted. Using in situ hybridization we show the presence of infected cells in the large and small intestine, mesenteric lymph nodes, spleen, and thymic organoid. We also demonstrate that infections virus can be isolated from these tissues. In this grant we propose to expand on these remarkable results and to establish the suitability of the BLT system to evaluate HIV transmission and its prevention by microbicides. With this in mind we propose the following aims for the R21 portion of this grant: Specific Aim 1) To characterize the systemic replication of R5 HIV-1 after intrarectal inoculation of BLT mice. Specific Aim 2) To evaluate the efficacy of a topical or systemic microbicide to prevent intrarectal transmission of HIV-1 in BLT mice. After completion of these two aims we propose to determine the susceptibility of BLT mice to intravaginal infection as indicated below in the specific aims for the R33 portion of this grant. Specific Aim 3) To evaluate the susceptibility BLT mice to intravaginal infection with cell-free X4 and R5 HIV-1. Specific Aim 4) To determine the efficiency of topical microbicides to prevent intravaginal HIV infection in the BLT model. Once we have established these basic parameters and at the same time determined the BLT's limitations we will be able to move forward with this novel system to contribute to the development and implementation of novel microbicides to prevent HIV transmission.